Imaging processes employing powdered toner are now very common. The toner may be applied as a dry powder or may be applied from a liquid. When applied from a liquid, the liquid portion does not transfer to the substrate in large amounts and solid toner particles carried by the liquid form a dry or damp powder image. To coalesce and bind the toner image to the substrate, one or more steps are taken, known collectively as fixing the image. Although various ways of fixing are known, such as the application of solvent, fixing by heat is very predominant in current technology. Fixing by heat avoids the addition of new materials to the system, which are a separate expense and which must be kept out of the atmosphere or otherwise kept from being an environmental hazard to the users.
However, heat fixing does not necessarily bind the powder firmly to the substrate and does not necessarily preserve well the image being fixed. Where ordinary paper is the substrate, the material of the toner, depending on its composition, may not flow sufficiently under heat to enter the fibers of paper and be firmly fixed. Toners of other compositions may flow too much into the paper and thereby lose edge definition and also appear gray rather than intense in color. Where the substrate has a continuous surface of organic material, such as polyester to function as a transparency, the toner, once again depending on its composition, may not bind well to the substrate or may wet the substrate and lose edge definition.
This invention employs properties of ionomeric resins to achieve exceptional imaging with heat fixing. lonomer resins are organic resins having polar substituents which are cross linked by metals between such substituents. They are known to be tough, scratch resistant, transparent, and readily melted by heat. U.S. Pat. Nos. 5,210,138 to Yamamoto et al and 4,968,752 to Kawamoto et al disclose ionomeric resins and their properties. The Yamamoto patent mentions their transparency and their use as packaging skins.
This invention may improve results for virtually any heat fixing application. Fixing of images is particularly difficult in full color systems, in which up to four layers of toner are accumulated (three primary colors and black) and then fixed. U.S. Pat. No. 5,291,255 to Britto et al and assigned to the same assignee to which this application is assigned, is illustrative of such a imaging system and is directed to heat fixing. Although the fixing is completed in that patent at the transfer step, fixing after the transfer step is clearly an alternative.
Ionomeric resins have been used for their special properties as resins of toners. U.S. Pat. No. 4,925,763 to Tsubuko et al discloses ionomeric resins for both liquid and dry toner and states that "the fixing performance of the toner particles increases as the fused toner particles are cooled and become hard, because of the intensified ionomeric bond." The toners of ionomeric resin are also said to be excellent in development performance.
U.S. Pat. No. 5,308,729, to Beach et al and assigned to the assignee of this invention employs a blend of an ionomeric resin and the acid form of that resin in a liquid toner application, and the "Background of the Invention" portion of that patent discloses other such teachings of the use of ionomeric resins. Images from such liquid developer are said to provide good-resolution print and fixing at relatively moderate temperature. Fuse grade, which is resistance to rubbing and scratching, is said to be good. U.S. Pat. No. 5,352,557 to Matsuoka et al is to a liquid developer employing ether liquid as the carrier liquid, and the resins are said to preferably have polar groups including "copolymers of acrylic acid and methacrylic acid or its ester and ethylene, or ionomer of the copolymers which are ionically crosslinked."
Use of an ionomer resin on the paper or other transfer sheet is not known to appear in the prior art.